I was in New York for a couple of days recently and as my N95-8GB (yes, I am a traditionalist) can only do UMTS 2100 MHz my voice calls and small screen Internet connectivity had to be content with the GSM network layer. But there are interesting things to discover here as well.
It looks like AT&T consistently uses AMR Half-Rate in the center of the city to double their voice capacity. All calls I established were always set-up with this codec. Also, AT&T uses both the 850 and 1900 MHz bands and if the mobile detects both, the network always handed the voice call over to a 1900 MHz carrier, even when the signal in the 850 MHz band was 30 db stronger. That could have many reasons, maybe they use the 1900 MHz band as a capacity layer and reserve the 850 MHz carriers for difficult terrain and indoor connections.
Speaking of difficult terrain: AT&T when you have a minute, have a radio team look at your coverage in Penn Station. Even when standing still, my calls in the underground station frequently dropped. Despite a high signal level, audio quality was horrible, which points to interference, and every couple of seconds, the call was handed over between the 850 and 1900 MHz layer and the traffic channel bounced between AMR Full-Rate and Half-Rate. Quite a frustrating experience.